LITTLE ROCK (AP) — After surrendering its supply of a lethal injection drug to federal agents in 2011, Arkansas turned to a somewhat surprising place to look for another drug: a list from lawyers for several death row inmates.

The state Department of Correction told The Associated Press last week that it decided to use phenobarbital after attorneys for several death row inmates mentioned in a lawsuit that it might be an available drug. Phenobarbital, which is used to treat seizures, has never been used in a U.S. execution, and critics contend that a drug that’s untested in lethal injections could lead to inhumane deaths for condemned prisoners.

“People should not be using inmates as an experiment,” said David Lubarsky, who chairs the anesthesiology department at the University of Miami’s medical school. “And that is basically what this is. It’s basically experimenting.”

As drugmakers object to their products’ use in lethal injections, more death penalty states have been looking at different options. But the states have revealed little, if any, information about how they go about picking drugs.

“It’s been: Here’s what we’re going to do and unless you can prove it’s excruciatingly painful, the courts, they’re not experts either, they’re going to allow it go forward,” said Richard Dieter, executive director of the Washington-based Death Penalty Information Center.

Just last year, Missouri announced plans to use propofol, the anesthetic blamed for pop star Michael Jackson’s death, even though that drug hadn’t been used in a lethal injection. In explaining the change, the state merely said the decision was due to a lack of another drug. Department of Corrections spokeswoman Mandi Steele declined to comment further Friday, saying the state’s execution protocol is the subject of legal action.

The changes came after states’ supplies of sodium thiopental dried up when the makers put it off-limits for use in executions. Oklahoma was the first to try pentobarbital, a sedative commonly used to euthanize animals, leading some other states to follow suit. But now those supplies are running out after manufacturers banned its use by states for executions.

In Arkansas, Department of Correction spokeswoman Shea Wilson said the agency consulted medical sources before picking phenobarbital, but Wilson refused to say what those sources were. She also wouldn’t say whether the agency considered using other drugs.

“Our research indicated the drug would take effect within 5 minutes and should result in a painless death,” Wilson said in an email to the AP.

But even the paperwork that came with the state’s supply of phenobarbital, which is a kind of drug known as a barbiturate, warns that the “toxic dose of barbiturates varies considerably,” according to records the AP obtained.

“We have no idea about whether or not the injection of such large doses will produce some acute tolerance effect, in which case you may or may not actually be able to kill someone with it,” said Lubarsky, who has testified in death penalty cases.

However, Mike Ritze, a family physician and Republican state lawmaker in Oklahoma, said phenobarbital is humane.

“I don’t want to compare humans with veterinarians, but for years, they euthanized animals and you can use anything in the barbiturates and they’re all very humane. Basically, the person just closes their eyes,” said Ritze, a proponent of the death penalty who said he has used phenobarbital to treat patients with seizures.

Arkansas and many of the nation’s more than 30 other death penalty states once used a virtually identical three-drug process: The barbiturate sodium thiopental was administered to put the inmate to sleep, and two other drugs were administered to stop breathing and the heart.

As sodium thiopental supplies dried up, Arkansas and several other states initially turned their attention overseas, obtaining the drug from a British supplier. But in 2011, they lost their supplies to federal agents amid legal questions about how they got the drug.

No one responded to phone messages left Friday at West-Ward Pharmaceuticals, the company from which Arkansas bought its latest batch of drugs.

Lubarsky, the anesthesiology department chair, said that “just because the drug sounds alike — thiopental, pentobarbital, phenobarbital — that doesn’t mean they’re that close a cousin.”

Those three drugs vary in the amount of time it takes for them to start kicking in. Sodium thiopental is like a light switch: It starts working quickly, and it wears off quickly. Phenobarbital, the drug Arkansas plans to use, takes a bit longer to kick in, and it takes longer for the drug to wear off.

Some say that means executions using phenobarbital could take more time.

Federal public defender Jenniffer Horan wrote to Arkansas Democratic Gov. Mike Beebe this week, calling on the governor to review the state’s planned use of phenobarbital, saying it carries a “substantial risk of a lingering and inhumane death.” A spokesman for Beebe said the governor’s office is reviewing the letter in conjunction with the attorney general’s office.

Arkansas hasn’t executed an inmate since 2005 and it doesn’t have any executions scheduled, though 37 inmates are on death row. Should executions be set, the state plans to inject inmates with the anti-anxiety drug lorazepam before giving them a large dose of phenobarbital.

“The lorazepam is a pre-execution sedative, but it also is intended to offset the side effects of the barbiturate should any develop,” Wilson said in an email.

However, Jon Groner, a surgery professor at the Ohio State University College of Medicine, says that might not work because lorazepam can make some people excitable, instead of relaxed. Groner has worked with lawyers who represent death row inmates and has written about how lethal injections create an ethical conflict he calls the Hippocratic paradox.

Medical professionals are arguably the most knowledgeable when it comes to the drugs used in lethal injections, but they’re supposed to heal people, not harm them.

“These states are not experts in pharmacology,” Dieter said. “They’re just trying to carry out their law.”